ch. 16: the digestive system. breakdown understand that digestion is a process and how that process...

Ch. 16: The Digestive System

Breakdown

• Understand that digestion is a process and how that process works

• Know the general anatomy of the digestive tract and it organs and accessory organs

• Know the physiology of digestion and absorption for the macromolecules as well as vitamins and minerals

Digestion

• What is digestion and why is it necessary?

• How is digestion accomplish?

Stages of Digestion• Ingestion

• Digestion– Mechanical and chemical

• Absorption

• Compaction

• Defecationhttp://www.ootonline.com/OOTnews/?p=1507

Digestive System

General Anatomy

• Structure: common layers through out the system

• Mucosa

• Submucosa

• Muscularis externa

• Serosa

Tissue Layers of GI Tract

Figure 25.2

Mucosa:

Lamina propriaMuscularis mucosae

Submucosa: Esophageal gland

Lumen

Blood vessels

Diaphragm

Esophageal hiatus

Myenteric plexus

Submucosal plexus

Muscularis externa: Inner circular layer

Outer longitudinal layer

Serosa

Enteric nervous system:

Stratified squamousepithelium

Parasympathetic ganglion ofmyenteric plexus

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Enteric Nervous System

• Regulates motility, secretion, blood flow

• Independent of CNS– CNS greatly influences

• Two networks– Submucosal plexus – Myenteric plexus

Regulation of Digestive Tract• Motility and secretion of the digestive tract are controlled by

neural, hormonal, and paracrine mechanisms

• Neural control– short (myenteric) reflexes – stretch or chemical stimulation acts through

myenteric plexus• stimulates parastaltic contractions of swallowing

– long (vagovagal) reflexes - parasympathetic stimulation of digestive motility and secretion

• Hormones– chemical messengers secreted into bloodstream– gastrin and secretin

• Paracrine secretions– chemical messengers that stimulate nearby target cells

Mouth

• Oral or buccal cavity

• Site of ingestion

• Teeth, tongue, saliva

• Mechanical and chemical digestionFigure 25.4

Copyright © The McGraw-Hill Companies, Inc.

The Teeth• Dentition – the teeth

• Masticate food into smaller pieces– makes food easier to swallow– exposes more surface area

• 32 adult teeth – 20 deciduous (baby) teeth– from midline to the rear of each jaw

• 2 incisors – chisel-like cutting teeth used to bite off a piece of food• 1 canine – pointed and act to puncture and shred food• 2 premolars – broad surface for crushing and grinding• 3 molars – even broader surface for crushing and grinding

Saliva• Saliva

– moisten mouth– begin starch and fat digestion– cleanse teeth– inhibit bacterial growth– dissolves molecules so they can stimulate the taste buds– moistens food and bind it together into bolus to aid in swallowing

• Hypotonic solution of 97.0% to 99.5% water and the following solutes:– salivary amylase – begins starch digestion in the mouth– lingual lipase – activated by stomach acid and digests fats– mucus – binds and lubricates the mass of food and aids in swallowing– lysozyme – enzyme that kills bacteria– immunoglobulin A (IgA) – an antibody that inhibits bacterial growth– electrolytes - Na+, K+, Cl-, phosphate and bicarbonate

• pH of 6.8 to 7.0

Esophagus• Esophagus – a straight muscular tube 25-30

cm long– extends from pharynx to cardiac orifice of

stomach – lower esophageal sphincter

• prevents stomach content regurgitation• heartburn – burning sensation produced by acid

reflux into the esophagus

– nonkeratinized stratified squamous epithelium– esophageal glands in submucosa secrete mucus– skeletal muscle in upper one-third, mixture in

middle one-third, and only smooth muscle in the bottom one-third

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Stomach• Stomach – muscular sac in upper left abdominal cavity

immediately inferior to the diaphragm– primarily functions as a food storage organ

• internal volume of about 50 mL when empty• 1.0 – 1.5 L after a typical meal• up to 4 L when extremely full and extend nearly as far as the pelvis

• Mechanically breaks up food particles, liquefies the food, and begins chemical digestion of protein and fat– chyme – soupy or pasty mixture of semi-digested food in

the stomach

Gross Anatomy of Stomach


© The McGraw-Hill Companies, Inc./Rebecca Gray, photographer/Don Kincaid, dissections

Microscopic Anatomy

• Mucosa – simple columnar epithelium

• Gastric pits – depressions in mucosa– Cardiac, pyloric and gastric

glands– Produce gastric juice

Figure 25.13 b-c


Cells of Gastric Glands• Mucous cells – secrete mucus

– predominate in cardiac and pyloric glands

• Regenerative (stem) cells – found in the base of the pit and in the neck of the gland– divide rapidly and produce a continual supply of new

cells

• Parietal cells – found mostly in the upper half of the gland– secrete hydrochloric acid (HCl), intrinsic factor,

hunger hormone ghrelin

• Chief cells – most numerous– secrete gastric lipase and pepsinogen– absent in pyloric and cardiac glands

• Enteroendocrine cells – concentrated in lower end of gland– secrete hormones and paracrine messengers that

regulate digestion Figure 25.13c

(c) Gastric gland

G cell

Mucous neck cell

Parietal cell

Chief cell


Hydrochloric Acid

• gastric juice has a high concentration of hydrochloric acid– pH as low as 0.8

• parietal cells produce HCl and contain carbonic anhydrase (CAH) CAH– CO2 + H2O H2CO3 HCO3

- + H+

– H+ is pumped into gastric gland lumen by H+- K+ ATPase pump• antiporter uses ATP to pump H+ out and K+ in

– HCO3- exchanged for Cl- (chloride shift) from blood plasma

• Cl- (chloride ion) pumped into the lumen of gastric gland to join H+ forming HCl• elevated HCO3

- (bicarbonate ion) in blood causes alkaline tide increasing blood pH

Figure 25.14

Blood Parietal cell Lumen of gastric gland

HCO3–

Cl–

H2CO3

H+

K+

Cl–

CO2

HCO3–

H2OCO2 +

H+–K+ ATPase

Stomachacid

Alkalinetide


Functions of Hydrochloric Acid• activates pepsin and lingual lipase

• breaks up connective tissues and plant cell walls– helps liquefy food to form chyme

• converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)– Fe2+ absorbed and used for hemoglobin synthesis

• contributes to nonspecific disease resistance by destroying most ingested pathogens

Production and Action of Pepsin

Figure 25.15


HCl

Parietal cell

Chief cell

Gastric gland

Pepsin(active enzyme)

Pepsinogen(zymogen)

Removedpeptide

Dietaryproteins

Partially digestedprotein

Gastric Secretions

• Gastric lipase– Combines with lingual lipase– Begin to digest small amount of fats

• Intrinsic factor– Secreted by the parietal cells– Required for vitamin B12 absorption– Only essential function of stomach

Stomach Rumbles

• Digestion and Absorption– Partial digestion of proteins and lipids– Absorption of aspirin and lipid-soluble drugs

• Protection – Mucus– Tight junctions– Cell replacement

Regulation of Gastric Function• Cephalic phase

– Stomach responds to sensory and mental info

• Gastric phase– Occurs after ingestion and deglutition– Stimulate gastric activity (secretion)

• Ach, Histamine, Gastrin– Act on parietal, chief, enteroendocrine cells

– Positive and negative feedback loops

• Intestinal phase– Initially stimulates gastric secrestions– Enterogastric reflex- inhibitory signals – Secretion of secretin and cholecystokinin

• Suppress gastric secretion and motility

Accessory Organs• Liver

– Produces and secretes bile

• Gall bladder– Stores and concentrates bile– Bile

• Contains cholesterol• Bile acids and lecithin help digest fat• 80% of bile acids reabsorbed in ilieum

• Pancreas– Exocrine tissue secretes pancreatic juice– Mixture of zymogens, enzymes, NaCO3, and water– Zymogens include: trypsinogen, chymotrypsinogen, procarboxypeptidase– Enzyms include: lipase, amylase, ribonuclease, deoxyribonuclease– Regulation

• Ach, CCK, Secretin

Small Intestine• Digestion and absorption completed

• Duodenum– Pancreatic enzyme digestion

• Jejunum– Highly vascularized– Absorption

• Ileum– Less vascularized– Absorption– Ileocecal valve


Intestinal Motility

• Segmentation

• Peristalsis– Migrating motor

complex


Large Intestine

• Begins with cecum, 1.5m long

• Ascending, transverse, descending, sigmoid

• Rectum

• AnusCopyright © The McGraw-Hill Companies, Inc.


Carbohydrate Digestion and Absorption

• Digestible dietary form is starch

• Primary enzyme – amylase– Digests starch into

oligosaccharides

• Secondary enzymes – dextrinase, glucoamylase, maltase, sucrase, lactase

Protein Digestion and Absorption• Three sources

– Dietary– Digested enzymes– Sloughed epithelial cells

• Digested by proteases– Pepsin (cleaves between tyrosine and phenylalanine)

• Stomach– Trypsin, chymotrypsin, carboxypeptidase

• Pancreatic secretions– aminopeptidase, dipeptidase

• Brush border enzymes

• Absorbed like carbohydrates

Figure 25.29

H N H

OC

HO

HN H

OC

HO

No chemical digestion occurs.

OC

HO

H N H

OC

HO

HN

H

OC

OH

H

HN

H

HN

OC

OH

OC

HO

HN H

OC

HO

HN H

OC

OH

H

HN

H

HN

OC

OH

OC

HO N

HN

H

CO OH

H

HN O

COH

H

HN

OC

OH

OC

OHH

HNH

N H

CO OH

OC

OH

H

HN

OC

OHH

HN

OC

OH

H

HN

OC

OH

H

HN O

COH

H

HN O

COH

H

HN

OC

OH

H

HN

OC

OH

H

HN O

COH

H

HN

Mouth

Stomach

Carboxypeptidase Aminopeptidase Dipeptidase

Protein

Protein

Polypeptides

Polypeptides Oligopeptides

Pepsin ( ) hydrolyzes certain peptide bonds, breaking protein down into smaller polypeptides.

Trypsin ( ) and chymotrypsin ( )hydrolyze other peptide bonds, breakingpolypeptides down into smalleroligopeptides.

Carboxypeptidase ( ) removes one amino acid at a time from the carboxyl (–COOH) end of an oligopeptide.

Carboxypeptidase ( ) of the brush border continues to remove amino acids from the carboxyl (–C OOH) end.

Aminopeptidase ( ) of the brush borderremoves one amino acid at a time from the amino (–N H ) end.

Dipeptidase ( ) splits dipeptides ( )into separate amino acids ( ).

Small intestineActions of brush border enzymes (contact digestion)

Blood capillaryof intestinal villus

Small intestineActions of pancreatic enzymes

Protein Digestionand Absorption


Lipid Digestion and Absorption• Digested by lipases

• Emulsification

• Micelles– Transport to brush border

• FFAs, monoglycerides, fat-soluable vitamins, cholesterol• Resynthesized

– Chylomicrons and exocytosis

• Enter lacteals

Emulsification

Figure 25.30

Emulsification

Bile acid

Lecithin Hydrophilic regionHydrophobic region

Fat globule

Emulsificationdroplets

Fat globule is broken up and coated bylecithin and bile acids.


Fat Hydrolysis and Micelles

Figure 25.30

Fat hydrolysis

Lipid uptake by micelles

Pancreatic lipasePancreatic lipase

Monoglyceride

Free fatty acid

Free fatty acid

Lecithin

Bile acid

Dietary lipid

Lipid core

MicellesFatty acids Fat-soluble vitamins

Bile acid

Monoglycerides Cholesterol

Emulsification droplets are acted upon bypancreatic lipase, which hydrolyzes thefirst and third fatty acids from triglycerides,usually leaving the middle fatty acid.

Micelles in the bile pass to the smallintestine and pick up several types ofdietary and semidigested lipids.

Triglyceride


Nucleic Acids and Vitamins • Nucleic acid

– nucleases (deoxyribonuclease and ribonuclease) hydrolyze DNA and RNA to nucleotides

– nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases

• Vitamins – are absorbed unchanged– fat-soluble vitamins - A, D, E and K absorbed with other

lipids• if they are ingested without fat-containing food, they are not absorbed at

all, but are passed in the feces and wasted

– water-soluble vitamins, B complex and C, absorbed by simple diffusion and B12 if bound to intrinsic factor from the stomach

Minerals

• minerals (electrolytes)– are absorbed all along small intestine

– Na+ cotransported with sugars and amino acids

– Cl- exchanged for bicarbonate reversing chloride-bicarbonate exchange that occurs in the stomach

– iron and calcium absorbed as needed• iron absorption is stimulated by liver hormone hepcidin• absorptive cells bind ferrous ions (Fe2+) and internalize by active

transport• unable to absorb ferric ions (Fe3+) but stomach acid reduces ferric

ions to absorbable ferrous ions• transferrin (extracellular protein) transports iron in blood to bone

marrow, muscle, and liver

Water Balance• Digestive system is one of several systems involved in water

balance• Digestive tract receives about 9 L of water/day

– 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions– 8 L is absorbed by small intestine and 0.8 L by large intestine– 0.2 L voided in daily fecal output

• Water is absorbed by osmosis following the absorption of salts and organic nutrients

• Diarrhea - occurs when large intestine absorbs too little water – feces pass through too quickly if intestine is irritated– feces contains high concentrations of a solute (lactose)

• Constipation – occurs when fecal movement is slow, too much water gets reabsorbed, and feces becomes hardened

ch. 16: the digestive system. breakdown understand that digestion is a process and how that process...

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